In general, a vibration-type angular velocity sensor has a movable part, that is, a vibrator that is movable in a direction along an X-axis (hereinafter, X-axis direction) and a direction along a Y-axis (hereinafter, the Y-axis direction).
The vibrator is regularly vibrated in the X-axis direction. When an angular velocity is applied around a Z-axis that is perpendicular to the X-axis and the Y-axis, the vibrator vibrates in the Y-axis direction due to Coriolis force caused by the angular velocity. The angular velocity sensor detects the angular velocity based on vibration of the vibrator in the Y-axis direction. Such a vibration-type angular velocity sensor is, for example, described in Japanese Patent Application Publication No. 2009-63328 (hereinafter, referred to as the publication 1).
In an angular velocity sensor, although a vibrator is vibrated in the X-axis direction, a vibration component in the Y-axis direction will be generated due to machining error of the vibrator and the like. That is, the vibrator may vibrate obliquely due to the machining error. In such an oblique vibration state, detection accuracy of the angular velocity is reduced.
Therefore, a servo control has been proposed in order to restrict the oblique vibration. In the servo control, an external force, such as an electrostatic force, in the Y-axis direction is generated by applying a predetermined driving signal to the vibrator.
A sensor unit having the vibrator is formed in a first semiconductor chip, and a detection unit for performing the servo control and detecting the angular velocity is formed in a second semiconductor chip. The first and second semiconductors are electrically connected to each other through a connecting portion, such as bonding wires or bumps formed in the semiconductor chips. The driving signal for performing the servo control is transmitted from the detection unit to the vibrator through the connecting portion.
If the connecting portion has disconnection, the angular velocity will not be accurately detected, as well as a malfunction will occur. Therefore, various means for detecting the disconnection have been proposed.
For example, Japanese Patent Application Publication No. 5-107292 (hereinafter, referred to as the publication 2) describes a disconnection detector for detecting disconnection of a signal line and a pair of power supply lines, which electrically connect between a sensor and an A/D converter for converting an output signal of the sensor into a digital value. In the disconnection detector of the publication 2, multiple resistors are connected between the signal line and each of the power source lines. If the signal line or the power source line has disconnection, an input voltage to the A/D converter becomes a different voltage level from a regular voltage level. The disconnection of the signal line or the power source lines can be detected based on the difference of the voltage level.
If the technique of the publication 2 is employed for detecting disconnection of the connecting portion of the vibration-type angular velocity sensor, the following drawbacks arise. In the nature of the servo control for reducing the oblique vibration of the vibrator, if the vibrator is ideally machined, the vibrator merely vibrates in the X-axis direction in a state where an angular velocity is not applied. That is, since the vibrator does not vibrate obliquely, it is not necessary to apply the external force by the servo control. Therefore, because the driving signal for the servo control does not pass through the connecting portion, the disconnection of the connecting portion cannot be detected.